Gerotor devices can be used in a variety of applications, one of the most common being to use the device in a low-speed, high-torque (LSHT) motor. Some gerotor devices can also be used in a high-speed, low-torque (HSLT) mode of operation. One example of a two-speed gerotor motor, one in which the motor is capable of operating in a LSHT mode of operation and an HSLT mode of operation, provides valving in the motor to recirculate fluid between expanding and contracting fluid pockets of a gerotor gear set, hereinafter referred to as a rotor set. In such a motor, if the inlet port of the motor communicates with all of the expanding pockets, and all of the contracting pockets of the rotor set communicate with the outlet port, the motor operates in a normal LSHT mode. To operate this known motor in the HSLT mode, the number of the contracting pockets and the expanding pockets is less than the LSHT mode.
Multiple speed gerotor motors have also been developed that include multiple rotor sets. Such a motor includes a first rotor set and a second rotor set, each defining a plurality of expanding and contracting fluid pockets. A selector valve is disposed between the first and second rotor sets. The motor is operable in an LSHT mode in which fluid flows from the fluid pockets in the first rotor set, then through the selector valve, then through the fluid pockets in the second rotor set. In an HSLT mode of operation, fluid out of the fluid pockets in the first rotor set is blocked by the selector valve, and fluid in the fluid pockets in the second rotor set flows through the selector valve to a case drain.
The two-speed gerotor motors that employ only one rotor set typically employ complicated valving mechanisms to change the number of expanding and contracting pockets. Alternatively, in two-speed gerotor motors where only one rotor set is employed, large end caps and valving mechanisms are employed making the motor much longer in an axial direction as compared to gerotor motors that operate in only the LSHT mode of operation. The hydraulic motors that employ more than one rotor set are also much longer than typical hydraulic motors that operate only in the HSLT mode of operation, and even longer than two-speed motors with one rotor set. These motors also include an additional rotor set, which can add significantly to the cost of the motor.